Conventionally, inkjet printers that perform printing by inkjet technology have been widely used (refer to, for example, non-patent document 1). The inkjet printer prints a color image by using four color inks (Y, M, C, and K inks) of Y (yellow) color, M (magenta) color, C (cyan) color, and K (black). As these inks, for example, pigment inks are widely used. As a method of printing by an inkjet printer, a method of printing in serial mode is widely used which includes causing inkjet heads to perform a main scanning operation (scanning operation). In this case, in order to print a color image, individual color ink droplets are usually ejected onto a medium by using the inkjet heads for the individual colors of Y, M, C, and K aligned in a moving direction (main scanning direction) of the inkjet heads during the main scanning operation.